Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed (or adsorber) which preferentially adsorbs a "heavy" or more readily adsorbed component relative to a "light" or less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorber from a first end to a second end of the adsorber, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A light product enriched in the light component is then delivered from the second end of the adsorber, while a heavy product enriched in the heavy component (often a waste stream) is exhausted from the first end of the adsorber. The light product is usually the desired product, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. Typically, the feed is admitted to the first end of an adsorber and the light product delivered from the second end of the adsorber when the pressure in that adsorber is relatively elevated, while the heavy product is exhausted from the first end of the adsorber at the low pressure of the cycle.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. This conventional pressure swing adsorption process also makes inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbers between higher and lower pressures.
The prior art also includes the following pressure swing adsorption devices with cyclically operated volume displacement means reciprocating at the same frequency at both ends of an adsorber, to generate pressure changes internally and thus improve energy efficiency.
Keller (U.S. Pat. No. 4,354,859) has disclosed a single adsorber pressure swing adsorption device for purifying both components of a binary gas mixture fed to a central point of the adsorber. This device has volume displacement means which may be pistons or diaphragms, of specified unequal displacements at opposite ends of the adsorber.
In our prior work, Keefer in U.S. Pat. No. 4,702,903 disclosed use of modified Stirling or Ericsson cycle machines for performing gas separations, in which expansion energy of the PSA cycle is recovered and heat may be applied directly through the modified Stirling cycle as a supplemental energy source to perform pressure swing adsorption gas separations. Keefer (U.S. Pat. No. 4,968,329) discloses related PSA gas separation devices with valve logic means to provide large exchanges of fresh feed gas for depleted feed gas. Such large feed exchanges, or effective scavenging, may be required when concentrating one component as a desired product without excessively concentrating or accumulating other components, as in concentrating oxygen from feed air containing water vapour whose excessive concentration and accumulation would deactivate the adsorbent. U.S. Pat. No. 5,082,473 (Keefer) discloses related multistage devices for with extraction and simultaneous concentration of trace components.
An important class of PSA device, as disclosed in the above cited prior patents by Keefer, uses reciprocating pistons communicating with both first and second ends of each adsorber to establish cyclic pressure variations in the adsorber, as well as establishing an oscillating flow pattern in the adsorber of flow in a first direction from first end to second end of the adsorber at the higher pressure, and flow in the reverse direction at the lower pressure. The principle of using positive displacement machinery to generate a high performance PSA cycle is referred to as "Thermally Coupled Pressure Swing Adsorption" or TCPSA, because of the inherent heat pumping aspect resulting from a close mechanical analogy to Stirling or Ericsson cycle thermodynamic engines.
All of the above cited devices use linear reciprocating pistons for establishing the cyclic pressure and reversing flow regime of PSA cycles. With relatively low PSA cycle frequencies attainable with conventional granular adsorbers, the reciprocating machinery is bulky and costly. One approach addressing this problem is use of rigid high surface area adsorbent supports which can overcome the limitations of granular adsorbent and enable much higher cycle frequencies. High surface area laminated sheet adsorbent supports, comprised of stacked or spirally wound adsorbent-impregnated sheet material, are disclosed in Keefer's U.S. Pat. Nos. 4,968,329 and 5,082,473.
Prior art PSA systems with multiport distributor valves have been used commercially in small scale oxygen enrichment applications, as recommended by Dangieri et al (U.S. Pat. No. 4,406,675) for a rapid PSA process in which flow control is intentionally established by relatively steep pressure gradients in the adsorber. The granular adsorber must therefore be spring-loaded or otherwise immobilized to prevent attritional damage.
For large industrial PSA systems, mechanical immobilization of the adsorbers has not been practicable. Careful flow control is required to ensure that pressure gradients in the adsorber are kept low, well below the onset of fluidization.
A further limitation to the use of finely granular adsorbers for PSA and other gas separation processes arises as increasingly smaller particle diameters are considered in order to reduce the macropore diffusion mass transfer resistance as required for higher frequency operation. It is well known (as outlined by D. M. Ruthven in "Principles of Adsorption and Adsorption Processes", Wiley, 1984, pages 210-211) that, owing to a tendency of very small particles to cluster and pack unevenly, "the advantage of reduced pore diffusional resistance which is gained by reduction of particle size can easily be offset by increased axial dispersion" for adsorbers packed of small particles.
As operation of PSA processes at high frequencies requires small particle sizes to reduce the diffusional time constant, while the increased axial dispersion prevents a reduction of adsorber length commensurate with smaller particle diameter, performance tends to degrade with high pressure drop and adsorber attrition problems. Hence, cycle frequencies much above 20 cycles per minute have been impracticable in sustained industrial applications, except by use of laminated sheet adsorbent support as mentioned above.
Mattia (U.S. Pat. No. 4,452,612), Lywood (U.S. Pat. No. 4,758,253), Boudet et al (U.S. Pat. No. 5,133,784), and Petit et al (U.S. Pat. No. 5,441,559) disclose PSA devices using a rotary adsorber configuration. The multiple adsorber ports of an adsorber rotor sweep past fixed ports for feed admission, product delivery and pressure equalization; with the relative rotation of the ports providing the function of a rotary distributor valve. Related devices are disclosed by Kagimoto et al (U.S. Pat. No. 5,248,325). All of these prior art devices use multiple adsorbers in parallel and operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbers.
An advantage of PSA devices with the adsorbers mounted on a rotary adsorber assembly, as in the cited prior art inventions by Mattia and Boudet et al., is that function port connections for feed, exhaust, product and pressure equalization are made to the stator and are thus accessible to flow control devices.
Within the above prior art, a rotary adsorber assembly may be impracticable for large PSA units, owing to the weight of the rotating assembly. Also, when separating gas components which are highly inflammable or toxic, the rotary adsorber assembly would need to be completely enclosed in a containment shroud to capture any leakage from large diameter rotary seals. Hence, PSA devices with stationary adsorbers have been hitherto preferred for larger scale systems, and for applications processing hazardous gases such as hydrogen.